Issue #276 - introduce class BSTR_Argument

src/plugin/COM_Value.cpp

+/**
+ * \file COM_Value.cpp Support for the values used in COM and Automation.
+ */
+#include "COM_Value.h"
+#include <stdexcept>
+
+using namespace AdblockPlus::COM;
+
+/*
+ * MSDN SysAllocStringLen function http://msdn.microsoft.com/en-us/library/windows/desktop/ms221639%28v=vs.85%29.aspx
+ */
+BSTR_Argument::BSTR_Argument(const std::wstring& s)
+{
+  auto length = s.length();
+  if (length == 0)
+  {
+    /*
+     * BSTR makes no distinction between null pointers and non-null pointers to the empty string.
+     * In order to detect allocation failure reliably, we simply don't allocate empty strings.
+     * Compare with the default constructor.
+     */
+    bstr = nullptr;
+    return;
+  }
+  // Assert length >= 1
+  bstr = SysAllocStringLen(s.c_str(), length);
+  /*
+   * Since the string is non-empty,
+   *   if the return value of SysAllocStringLen is null it means that it had an allocation failure.
+   */
+  if (!bstr)
+  {
+    throw std::bad_alloc();
+  }
+}
+
+/*
+ * MSDN SysFreeString function http://msdn.microsoft.com/en-us/library/windows/desktop/ms221481%28v=vs.85%29.aspx
+ */
+BSTR_Argument::~BSTR_Argument()
+{
+  // There's no need to check for null; SysFreeString already handles that case.
+  SysFreeString(bstr);
+}
+
+BSTR* BSTR_Argument::operator&()
+{
+  // Assign the empty string, if not already empty.
+  if (bstr)
+  {
+    SysFreeString(bstr);
+    bstr = nullptr;
+  }
+  return &bstr;
+}
+
+BSTR_Argument::operator std::wstring() const
+{
+  if (!bstr)
+  {
+    return std::wstring();
+  }
+  return std::wstring(bstr, SysStringLen(bstr));
+}